This invention relates to writing compositions, instruments, and methods and, more particularly, to erasable, shear-thinning writing compositions.
Erasable, graphite-containing liquid writing compositions have been proposed for use in writing instruments to combine the features of pens and pencils. Writing instruments including these liquid graphite-containing compositions are similar both to pens, in that they require no sharpening, no advancing, and low application pressure, and to pencils, in that they can be used to make markings on a substrate, such as paper, which subsequently can be erased from the substrate.
According to one aspect of the invention, an erasable, shear-thinning writing composition includes a shear-thinning additive and graphite particles in a liquid.
According to another aspect of the invention, an erasable, shear-thinning writing composition includes a shear-thinning additive and graphite particles dispersed in a liquid, wherein the composition has a shear-thinning index between about 0.01 and 0.8.
In accordance with another aspect of the invention, a writing instrument includes a reservoir containing an erasable, shear-thinning writing composition comprising an aqueous solvent system, a shear-thinning additive, and graphite particles, wherein the graphite particles and the shear-thinning additive are dispersed within the aqueous solvent system. The writing instrument further includes a writing point in fluid communication with the reservoir.
In accordance with an additional aspect of the invention, a method of forming a marking includes making a marking on a substrate with an erasable, shear-thinning writing composition comprising a shear-thinning additive and graphite particles.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description and appended claims.
One aspect of the invention is an erasable, shear-thinning writing composition comprising a shear-thinning additive and graphite particles in a liquid.
Another aspect of the invention is an erasable, shear-thinning writing composition including a shear-thinning additive and graphite particles dispersed in a liquid wherein the erasable, shear-thinning composition has a shear-thinning index between about 0.01 and about 0.8.
Additional embodiments of these aspects may include one or more of the following features. The erasable, shear-thinning writing composition can have a shear-thinning index between about 0.01 and about 0.8. The erasable, shear-thinning composition also has a viscosity between 200 mPaxc2x7sec and 20,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921, and a viscosity between 10 mPaxc2x7sec and 1000 mPaxc2x7sec at a shear rate of about 100 secxe2x88x921. The liquid is an aqueous solvent system. The shear-thinning additive and graphite particles are dispersed within the aqueous solvent system. The shear-thinning additive is a clay, an organoclay, a water-dispersible gum, an acrylic acid-based polymer, or a combination thereof. The clay is a hectorite clay or a bentonite clay. The water-dispersible gum is a polysaccharide, such as, for example, xanthan gum. The composition can include about 0.01 weight percent to about 10 weight percent of the shear-thinning additive, about 0.05 weight percent to about 5 weight percent of the shear-thinning additive, or about 0.1 weight percent to about 2 weight percent of the shear-thinning additive. The composition further comprises at least one water-soluble organic solvent. The composition further comprises at least one water-soluble organic solvent selected from the group consisting of glycols, polyhydric alcohols, glycol ethers, glycol ether esters, amines, amides, and alkanolamides. The water-soluble organic solvent can be a glycol, such as, for example, propylene glycol. The aqueous solvent system comprises water, propylene glycol, and glycerol. The water of the aqueous solvent system is deionized water. The mixture includes from about 0.5 part to about 25 parts water per part water-soluble organic solvent(s). The graphite particles can comprise from about 1 weight percent to about 50 weight percent of the composition, from about 3 weight percent to about 25 weight percent of the composition, or from about 5 weight percent to about 20 weight percent of the composition. The graphite particles have an average thickness of less than about 1 micron, and an average particle diameter between about 1 micron and about 25 microns, or an average thickness of less than about 0.5 micron, and an average particle diameter between about 2 microns and about 15 microns, or an average thickness of less than about 0.25 microns, and an average particle diameter between about 3 microns and about 12 microns. The graphite particles are selected from the group consisting of amorphous graphite, flake natural graphite, primary synthetic graphite, secondary synthetic graphite, and combinations thereof. The erasable, shear-thinning composition further includes one or more dispersants. The dispersant is a water-soluble polymer. The erasable, shear-thinning writing compositions can exhibit an erasability greater than about 80 percent and a line intensity greater than about 25 percent, or an erasability greater than about 95 percent and a line intensity greater than about 30 percent, or an erasability greater than 97 percent and a line intensity greater than about 40 percent. The composition further comprises pigment particles.
Another aspect according to the invention is a writing instrument comprising a reservoir containing an erasable, shear-thinning writing composition comprising an aqueous solvent system, a shear-thinning additive, and graphite particles wherein the graphite particles and the shear-thinning additive are dispersed within the aqueous solvent system. The writing instrument further contains a writing point in fluid communication with the reservoir. The writing instrument can be a pen, such as, for example, a ballpoint pen.
Yet another aspect according to the invention is a method of forming a marking comprising making a marking on a substrate with an erasable, shear-thinning writing composition. The composition comprises a shear-thinning additive and graphite particles.
Embodiments of this aspect of the invention may include one or more of the following features. The method further comprises erasing the marking from the substrate. The erasable, shear-thinning composition further comprises an aqueous solvent system. The erasable, shear-thinning composition has a shear-thinning index between about 0.01 and about 0.8. The erasable, shear-thinning writing composition has a viscosity between 200 mPaxc2x7sec and 20,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921, and a viscosity between 10 mPaxc2x7sec and 1000 mPaxc2x7sec at a shear rate of about 100 secxe2x88x921. The substrate is paper. The marking is made with a writing instrument, such as a ballpoint pen.
In another aspect, the invention features an erasable, shear-thinning writing composition including about 30 weight percent to about 99 weight percent of an aqueous solvent system, about 0.01 weight percent to about 10 weight percent of a shear-thinning additive, and about 1 weight percent to about 50 weight percent of graphite particles dispersed within the solvent system.
In other aspects, the invention can also feature an erasable, shear-thinning writing composition including a shear-thinning additive, graphite particles, and pigment particles in a liquid. Examples of pigment particles include, but are not limited to, pearlescent pigments, such as, for example, Afflair 110, available from EM Industries, Inc., located in Hawthorne, N.Y.
Shear-thinning compositions are non-Newtonian liquids that exhibit shear-thinning flow behavior when subjected to shear. Preferred shear-thinning compositions of the invention become thin, readily flowable liquids having a viscosity of no greater than about 500 mPaxc2x7sec at shear rates greater than about 100 secxe2x88x921. Typically, the erasable, shear-thinning composition has a viscosity between 200 mPaxc2x7sec and 20,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921, and a viscosity between 10 mPaxc2x7sec and 1000 mPaxc2x7sec at a shear rate of 100 secxe2x88x921. The erasable, shear-thinning writing compositions of this invention generally include at least one water dispersible, shear-thinning additive dispersed in an aqueous solvent system.
In preferred implementations, the graphite particles may include amorphous graphite, flake natural graphite, primary synthetic graphite, secondary synthetic graphite, or combinations thereof. The graphite particles can be dispersed in a solvent.
When used in a writing instrument, preferred erasable, shear-thinning writing compositions exhibit even laydown. The erasable, shear-thinning compositions are erasable by common erasers, such as Pink Pearl erasers, available from Sanford Corporation, located in Bellwood, Ill., and provide good writing performance, e.g., a line intensity greater than about 25 percent, more preferably greater than about 30 percent, and most preferably greater than about 40 percent. Additionally, the graphite particles remain suspended in the composition during storage, minimizing or even eliminating the need to mix or agitate the composition prior to producing a marking of a substrate.
As used herein, the term xe2x80x9claydownxe2x80x9d refers to the amount of writing fluid (e.g., shear-thinning writing composition) that is deposited on a substrate when making a marking of a particular length. Typically, the laydown for the preferred shear-thinning writing compositions according to the invention is between 0.1 mg/m and 8.0 mg/m; preferably, between 0.5 mg/m and 5.0 mg/m; and most preferably, between 1.5 mg/m and 3.5 mg/m.
As used herein, the term xe2x80x9ceven laydownxe2x80x9d refers to the morphology of the writing fluid when applied to a substrate to create a continuous marking and is characterized by minimal skipping, i.e., few voids within the written line, and uniform thickness, i.e., the width of the written line is approximately constant along the length of the line.
As used herein, the term xe2x80x9cline intensityxe2x80x9d refers to the intensity of a marking made on a substrate such as, for example, paper. The intensity of a marking can be measured as the average gray value of the detected tracings (black=0; white=255). The percent intensity of the writing with an average gray value of z is then calculated as: %Intensity=(1xe2x88x92[z/255]) multiplied by 100. Alternatively, the intensity of a marking can be determined by calculating the difference between the recorded reflectance of the substrate without any marking (xe2x80x9cBlank Reflectancexe2x80x9d) and the reflectance of the marking on the substrate (xe2x80x9cReflectance of Markingxe2x80x9d). According to this method, the percent intensity of a marking is calculated by normalizing the calculated intensity difference to the Blank Reflectance and multiplying this value by 100. The data obtained from these two methods are comparable.
As used herein, the term xe2x80x9cline uniformityxe2x80x9d refers to the standard deviation of the line intensity measured along different portions of a marking made on a substrate. Line uniformity can be used as a measure of even laydown.
As used herein, the term xe2x80x9cerasabilityxe2x80x9d refers to the ability to recover the gray level reading of the blank paper by removing the written tracings with an eraser. The percent erasability can be calculated as: %Erasability=(z/zo) multiplied by 100 where z is the average gray value of the erased section and zo is the average gray value of the blank section of paper. Alternatively, the erasability can be determined by recording the reflectance of each erased line (xe2x80x9cReflectance of Erased Linexe2x80x9d) and the reflectance of the paper without any marking (xe2x80x9cBlank Reflectancexe2x80x9d) and calculating the ratio of Reflectance of Erased Line to Blank Reflectance, i.e., Etot=(Erased Line/Blank). According to this method, the percent erasability is calculated by multiplying Etot by 100. The data obtained from these two methods are comparable.
As used herein, the terms xe2x80x9cpigmentxe2x80x9d and xe2x80x9cpigment particlesxe2x80x9d are meant to encompass materials other than graphite and/or graphite particles.
Erasable, shear-thinning writing compositions according to the invention include a shear-thinning additive and graphite particles both dispersed in an aqueous solvent system. Typically, the composition includes from about 0.01 weight percent to about 10.0 weight percent of the shear-thinning additive, from about 1 weight percent to about 50 weight percent of graphite particles, and from about 30 weight percent to about 99 weight percent of the aqueous solvent system. Preferably, the compositions include from about 0.05 weight percent to about 5 weight percent shear-thinning additive, from about 3 weight percent to about 25 weight percent graphite particles, and from about 60 weight percent to about 96 weight percent aqueous solvent system. Most preferably, the composition includes from about 0.1 weight percent to about 2 weight percent shear-thinning additive, from about 5 weight percent to about 20 weight percent graphite particles, and from about 74 weight percent to about 94 weight percent aqueous solvent system.
Shear-thinning Additives:
Suitable shear-thinning additives are miscible or dispersible in the aqueous solvent along with the dispersed graphite particles, and provide an erasable, shear-thinning writing composition having a shear-thinning index (n) of between about 0.01 and about 0.8, preferably between about 0.05 and about 0.60, and most preferably between about 0.1 and about 0.3. The shear-thinning index (n) is determined by fitting the shear stress (xcfx84) and shear rate (xcex3) values obtained from rheological measurements to the empirical power law equation: xcfx84=Kxcex3n wherein the coefficient (K) is a constant. The exact value of K depends on the composition being tested. The shear-thinning index is also described in U.S. Pat. No. 4,671,691, the disclosure of which is incorporated herein by reference. Shear stress values are measured continuously from 0.5 secxe2x88x921 to 1000 secxe2x88x921 and are fit to the power law model to determine the shear-thinning index. Shear-thinning measurements can be performed on a rheometer, such as a Carri-Med Rheometer CSL2-500, available from TA Instruments, located in New Castle, Del.
Suitable shear-thinning additives provide erasable, shear-thinning writing compositions which are thickened viscous liquids at rest or at low shear rates. In general, the viscosity decreases as the shear rate increases. Typically, erasable, shear-thinning writing compositions have a viscosity between 200 mPaxc2x7sec and 20,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921; preferably, the shear-thinning writing compositions have a viscosity between 2000 mPaxc2x7sec and 18,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921; and, most preferably, the shear-thinning writing compositions have a viscosity between 5000 mPaxc2x7sec and 15,000 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921. Typically, erasable, shear-thinning writing compositions have a viscosity between 10 mPaxc2x7sec and 1000 mPaxc2x7sec at a shear rate of 100 secxe2x88x921; preferably, the shear-thinning writing compositions have a viscosity between 50 mPaxc2x7sec and 700 mPaxc2x7sec at a shear rate of 100 secxe2x88x921; and, most preferably, the shear-thinning writing compositions have a viscosity between 100 mPaxc2x7sec and 500 mPaxc2x7sec at a shear rate of 100 secxe2x88x921. As a result, the shear-thinning additives provide an erasable, shear-thinning writing composition having a shear-thinning index (n) between about 0.01 and about 0.8, a viscosity greater than 200 mPaxc2x7sec at a shear rate of about 1.0 secxe2x88x921, and a viscosity less than about 1000 mPaxc2x7sec at shear rates above 100 secxe2x88x921.
Suitable shear-thinning additives do not interact to any significant extent with the substrate materials, e.g., paper, on which the erasable, shear-thinning writing composition is used, in a manner that would deleteriously affect erasability. Suitable shear-thinning additives include, but are not limited to, clays, such as smectites (bentonite and hectorite), and to organoclays, typically smectites modified with long chain organic cation groups. The term xe2x80x9csmectitexe2x80x9d refers to a family of non-metallic clays that are primarily composed of hydrated sodium calcium aluminum silicate, including bentonite and hectorite. Common names for smectites include montmorillonite or sodium montmorillonite (xe2x80x9csodium bentonitexe2x80x9d or xe2x80x9cWyoming bentonitexe2x80x9d) and swelling bentonite (xe2x80x9cWestern bentonitexe2x80x9d). Bentonite is a native, colloidal, hydrated, non-metallic mineral of the dioctahedral smectite group, is primarily composed of the mineral montmorillonite, and has been processed to remove grit and non-swellable ore components. Hectorite is a native, colloidal mineral of the trioctahedral smectite group and is primarily composed of sodium magnesium lithium silicate. Typically, hectorite is processed to remove grit and impurities. Clays, such as Bentone MA, and organoclays, such as Bentone 34, are available from Rheox Inc., Hightstown, N.J.
Other suitable shear-thinning additives include water-dispersible gums or resins which can be either natural or synthetic. Natural gums include seaweed extracts, plant exudates, seed or root gums and microbiologically fermented gums. Synthetic gums, such as modified versions of cellulose or starch, include propylene glycol alginate, carboxymethyl locust bean gum and carboxymethyl guar. Many water-dispersible gums can also be described as polysaccharides, because their structure consists of repeating sugar units. Examples of water-dispersible gums include, but are not limited to, xanthan gum (Keltrol and Kelzan made by Kelco Biopolymers, San Diego, Calif.), carboxymethylcellulose (sold as a sodium salt, Blanose, by Hercules Incorporated, Wilmington, Del.), hydroxyethylecellulose (Natrosol, manufactured by Hercules; Cellosize, by Union Carbide Corporation, Danbury, Conn.), sodium alginate and other salts of alginic acid, kappa, iota and lambda carrageenan (sulfated polysaccharides extracted from red seaweed), gum arabic (mixed salts of arabic acid), gum karaya (an acetylated polysaccharide), gum tragacanth (a complex mixture of acidic polysaccharides), gum ghatti (the calcium and magnesium salt of a complex polysaccharide), guar gum (a straight chain galactomannan) and its derivatives (Jagar, manufactured by Rhodia, Inc., Cranbury, N.J.), locust bean gum (a branched galactomannan), tamarind gum, psyllium seed gum, quince seed gum, larch gum, pectin and its derivatives, dextran, hydroxypropylcellulose (Klucel, manufactured by Hercules), cellulose ethers (Methocel, manufactured by Dow Chemical Company, Midland, Mich.) and other water-soluble gums of this type.
Other suitable shear-thinning additives include high molecular weight homo- and copolymers of acrylic acid crosslinked with polyalkenyl polyether sold by B F Goodrich, Charlotte, N.C., under the tradename Carbopol, e.g., Carbopol 934, 940, and 941. Carbopol homopolymers are polymers of acrylic acid crosslinked with allyl sucrose or allylpentaerythritol, and Carbopol copolymers are polymers of acrylic acid modified by long-chain (C10-C30) alkyl acrylates and crosslinked with allylpentaerythritol. Carbopol polymers, also called Carbomers, typically have high molecular weights between about 350,000 and 5,000,000.
Graphite Particles:
Suitable graphite particles include, but are not limited to, amorphous graphite, flake natural graphite, primary synthetic graphite, and secondary synthetic graphite. Primary and secondary synthetic graphite particles are synthetically produced and purified particles, whereas amorphous and flake graphite particles are naturally occurring. Preferably, the graphite particles are flake natural graphite. Preferably, the graphite particles have an average thickness less than about 1 micron, and an average particle diameter between about 1 micron and about 25 microns; more preferably, the average thickness is less than about 0.5 micron, and the average particle diameter is between about 2 microns and about 15 microns; and most preferably, the average thickness is less than about 0.25 micron, and the average particle diameter is between about 3 microns and about 12 microns.
The dimensions of the graphite particles can also be described by an aspect ratio of the length to the width. The average length and average width can be the same or different. Typically, the average width of the graphite particles is less than the average length. An aspect ratio of the length to the width, typically, is between about 1 and about 8; preferably, between about 1 and about 3; and most preferably, between about 1 and about 2. The average dimensions of graphite particles can be ascertained by performing scanning electron microscopy (SEM). Furthermore, typically more than 90 percent of the graphite particles have a diameter between 1 micron and 20 microns; more preferably, more than 95 percent of the graphite particles are between 1 micron and 20 microns; most preferably, more than 98 percent of the graphite particles are between 1 micron and 20 microns.
In general, the largest dimension of the graphite particles is limited by the need to pass through the point openings in writing instruments and by the requirement to form stable suspensions that do not settle over time. The smallest dimension of the graphite particles is selected to limit penetration of the particles into the interstices of the substrate material. The flake-like morphology of the graphite particles results in a xe2x80x9cleafingxe2x80x9d phenomenon wherein the particles lie flat and align horizontally on the surface of the substrate material, overlapping each other, without penetrating into the interstices of the substrate. Such leafing particles are easily erased, whereas particles in the interstices generally are not. Graphite particles having a spherical or cube-like morphology tend not to exhibit this leafing tendency, which can reduce erasability as well as deleteriously affect the laydown properties of the erasable, shear-thinning writing compositions. Highly structured carbon black particles also exhibit a tendency to penetrate into the interstices of the substrate. Examples of suitable graphite particles include, but are not limited to, those sold under the tradenames, Micro750 and Micro790 (flake), Micro150 and Micro190 amorphous), Micro250 and Micro290 (primary synthetic), and Micro450 and Micro490 (secondary synthetic), available from Graphite Mills, Inc. (Asbury Graphite Mills, N.J.)
Because of the flexibility of the graphite particles, the deposited writing compositions form elastic films on drying. The integrity of these films is strong enough to prevent flaking when the paper substrate is bent or folded. As a result of the inherent elastic nature of the films, the shear-thinning writing compositions according to the invention do not normally need an agent specifically for forming films. Nonetheless, a small amount of film-forming can be added if smudging of the written material is a concern. Smudging tends to increase with increasing laydown and with increasing particle size. However, as the concentration of film-forming agent increases, the erasability of the writing composition decreases. Eventually, enough film-forming agent can be added to the writing composition to make it non-erasable. Examples of suitable film-forming agents for use in the writing compositions according to the invention include, but are not limited to, acrylic copolymers such as Avalure AC 120 and Avalure 122, and polyurethane dispersions, such as Avalure UR 425 and Avalure 450, all of which are available from B F Goodrich Performance Materials, Cleveland, Ohio.
Other Pigment Particles:
xe2x80x9cGraphite-color pigments,xe2x80x9d which are flake-like pigments that provide graphite-like color, can also successfully be used in the erasable, shear-thinning writing compositions according to the invention. Typically, such pigments have been surface-modified by chemical reaction, or by adsorption, or by coating with other colorants (pigments and/or dyes.) Examples of graphite-color pigments that can be used in the compositions according to the invention include, but are not limited to, aluminum flakes, mica flakes, and bismuth oxychloride flakes. Suitable aluminum flakes include, for example, Metalure, Alucolor (organic pigment/aluminum), and Aloxal (aluminum with oxidized surface), available from Eckart America, L. P., Painesville, Ohio. Suitable mica flakes include, for example, Black Mica (iron oxide, titanium dioxide/mica), Micronasphere M (silica/mica), Colorona Blackstar Blue (iron oxide/mica), Microna Matte Blue (ferric ferrocyanide/mica), and Afflair 110 (titanium dioxide/mica), available from EM Industries, Inc., Hawthorne, N.Y. Suitable bismuth oxychloride flakes include, for example, Biron ESQ and Biron LF-2000, also available from EM Industries, Inc. A dark pigment must be mixed with Alucolor, Aloxal, Micronasphere M, Afflair 110, Biron ESQ, and Biron LF-2000 in order to achieve the desired graphite color. The presence of some of these pigments, such as Afflair 110, in the compositions according to the invention provides a pearlescent sheen to the surface of the deposited writing composition.
Aqueous Solvent System:
The aqueous solvent system of the erasable, shear-thinning writing composition is a polar solvent system in which water is the primary solvent. The aqueous solvent system can consist of water alone, but other water-soluble organic solvents which are useful in inhibiting drying in the point of the writing instrument and in preventing the shear-thinning writing composition from freezing at low temperatures can be included in the aqueous solvent system. Typically, the shear-thinning writing composition includes from 1 percent by weight to 40 percent by weight of a water-soluble organic solvent. Preferably, the shear-thinning writing composition includes 5 percent by weight to 30 percent by weight of a water-soluble organic solvent. Most preferably, the shear-thinning writing composition includes about 8 percent by weight to 25 percent by weight of a water-soluble organic solvent. If too much water-soluble organic solvent is added to the shear-thinning writing composition, the written marks take longer to dry, have worse erasability, exhibit poorer writing characteristics (uneven line intensity), and the solubility of the shear-thinning agent in the formulation may be affected.
The aqueous solvent system can be described in terms of the ratio of water to water-soluble organic solvent. For example, the polar solvent system can be a 1:1 mixture of water and a water-soluble organic solvent. Typically, the ratio of water to water-soluble organic solvent is from about 0.5 part to about 25 parts water per part of organic solvent(s). Preferably, the ratio of water to water-soluble organic solvent is from about 1 part to about 20 parts water per part of organic solvent(s). Most preferably, the ratio of water to water-soluble organic solvent is from about 2 parts to about 10 parts water per part of organic solvent(s). In general, preferred ratios of water to water-soluble organic solvents lead to better erasability and writing characteristics such as even laydown and line intensity.
Examples of water-soluble organic solvents include, but are not limited to, glycols, polyhydric alcohols, glycol ethers, glycol ether esters, amines, amides, and alkanolamides. Other examples of water-soluble organic solvents can be found in McCutcheon""s Volume 2: Functional Materials, North American Edition; McCutcheon""s Division, The Manufacturing Confectioner Publishing Co., Glen Rock, N.J. (1998), the disclosure of which is herein incorporated by reference. Examples of glycols include, but are not limited to, ethylene glycol, diethylene glycol, propylene glycol, triethylene glycol, polyethylene glycol, and polypropylene glycol. Examples of polyhydric alcohols include, but are not limited to, sorbitol, glycerol, diglycerol, and triglycerol. Examples of glycol ethers include, but are not limited to, ethylene glycol monophenyl ether, diethylene glycol monomethyl ether, propylene glycol monomethyl ether, and dipropylene glycol monomethyl ether. An example of a suitable glycol ether ester is, but is not limited to, propylene glycol monomethyl ether acetate. Examples of amines include, but are not limited to, ethanolamine, diethanolamine, and triethanolamine. Examples of amides include, but are not limited to, urea and thiourea. Examples of alkanolamides include, but are not limited to, Acetamide MEA (Witco Corporation, Greenwich, Conn.) and Schercomid AME-70 (Scher Chemicals, Inc., Clifton, N.J.).
Dispersants:
Additionally, the density and the size of the graphite and other pigment particles in the writing composition necessitate the use of one or more effective dispersants to disperse the particles into the shear-thinning writing composition. Typically, such dispersants are water-soluble polymers that include polymeric chains having xe2x80x9canchoring groupsxe2x80x9d which may or may not carry a charge, and which are attracted to the graphite and/or pigment particulate surface. When the unbound portion of the polymeric chain is well solvated, it helps to stabilize the dispersion of particles in the solvent system. Dispersants are also used to reduce the drying times of the erasable, shear-thinning composition. Typically, the shear-thinning writing composition includes about 0.01 percent by weight to 5 percent by weight of one or more suitable dispersants; preferably, between about 0.02 percent by weight to 4 percent by weight of one or more dispersants; and most preferably, between about 0.05 percent by weight and 2 percent by weight of one or more dispersants. Compositions not containing sufficient amounts of one or more dispersants may show poor writing performance (reduced or no flow from the point), and may exhibit poor stability with time and/or elevated temperature.
Examples of suitable dispersants include, but are not limited to, nonionic copolymers such as Disperbyk-192 (BYK-Chemie USA, Inc., Wallingford, Conn.), anionic copolymers such as Disperbyk-190 and Disperbyk-191 (BYK-Chemie USA, Inc., Wallingford, Conn.), anionic phosphated alkoxylated polymers such as Solsperse 40000 and Solsperse 41090 (Avecia Pigments and Additives, Charlotte, N.C.), anionic dimethicone copolyol phosphates such as Pecosil PS-100 and Pecosil PS-150 (Phoenix Chemical, Inc., Somerville, N.J.) and other polymers such as Zephrym PD2434, Zephrym PD2630, Zephrym PD2678, and Zephrym PD3076, available from Uniquema, Wilmington, Del.
Wetting Agents:
In order to produce a consistent written line, the formulation must readily wet the ball of the writing instrument. Furthermore, the formulation must also wet the paper so that written marks dry fast by absorption of the solvent into the paper. Preferred wetting agents can be either anionic or nonionic. Typically, the shear-thinning writing composition includes about 0.01 percent by weight to 5 percent by weight of one or more suitable wetting agents; preferably, between about 0.02 percent by weight to 4 percent by weight of one or more wetting agents; and most preferably, between about 0.05 percent by weight and 2 percent by weight of one or more wetting agents.
Examples of suitable wetting agents include, but are not limited to, anionic phosphate esters such as Ethfac 324 and Ethfac 361 (Ethox Chemical, LLC, Greenville, S.C.), anionic sulfosuccinates such as Emcol 4100M (Witco Corporation, Greenwich, Conn.) and Triton GR-5M (Union Carbide Corporation, Danbury, Conn.), nonionic ethoxylated fatty acids such as Emerest 2634 and Emerest 2646 (Cognis Corporation, Cincinnati, Ohio), nonionic ethoxylated alcohols such as Brij 58, Brij 98, Renex 20, Renex 36 and Synthrapol KB (Uniquema, Wilmington, Del.), and nonionic polyether-modified polydimethylsiloxanes such as BYK-345, BYK-348, BYK-307 and BYK-333 (BYK-Chemie USA, Inc., Wallingford, Conn.).
Preservatives:
Shear-thinning writing compositions thickened with polysaccharide gums require the use of one or more preservatives to prevent the growth of bacteria and fungi. Preferred compounds have an active moiety which is anionic. The preferred agent is a broad-spectrum biocide, 1,2,-benzisothiazolin-3-one, sold as a solution or dispersion under the tradename Proxel. Examples of suitable preservatives include, but are not limited to, Proxel GXL, Proxel BD20, and Proxel XL2 (Avecia Biocides, Wilmington, Del.) Typically, the shear-thinning writing compositions according to the invention can include 0.01 percent by weight to 0.05 percent by weight of the active ingredient in the preservative product. Other preservatives include, but are not limited to, potassium sorbate, sodium benzoate, pentachlorophenyl sodium, and sodium dihydroacetate.
Other Additives:
The shear-thinning writing compositions according to the invention can also include other additives that are well-known in the art, such as defoamers, corrosion inhibitors, and lubricants.
Additionally, the pH of the composition can be adjusted to increase the stability and writing characteristics of the writing composition. For example, the stability of erasable, shear-thinning writing compositions may be enhanced by adjusting the pH of the composition to between about 5 and about 9, e.g., by adding an acid or a base. More preferably, the pH of the graphite writing composition is between about 7 and about 9 and, most preferably, the pH of the graphite writing composition is between about 7 and about 8.
Writing Instruments:
Suitable writing instruments to deliver the erasable, shear-thinning compositions include, but are not limited to, conventional ballpoint pens. The tip of a ballpoint pen suitable for use with compositions according to the invention has a ball having a diameter between 0.3 mm and 2.0 mm. The ball is in direct contact with a fluid reservoir containing the writing composition. The clearance between the point opening and the ball must be of sufficient size to allow the graphite particles of the erasable, shear-thinning writing compositions according to the invention to pass through the point. Preferably, the clearance is at least about 100 microns; more preferably, at least about 25 microns. The ball is made from a group of materials which includes sintered hard alloys, ceramics, and resins. The point material is selected from materials including stainless steel, nickel silver, brass, and molded resins. The point can also contain a spring which contacts the ball and presses it against the inner edge of the front of the point until the force of writing pushes the ball back. Such ballpoint pens having a spring are described in U.S. Pat. No. 5,929,135, the entire disclosure of which is herein incorporated by reference. Other examples of ballpoint pens (without springs) which may be used with the writing composition are the PaperMate Gel Stick pen (Sanford, Bellwood, Ill.) and the uni-ball Signo gel ink pen (Mitsubishi Pencil Co., Ltd., Japan).
The invention can be better understood in light of the following examples which are intended as an illustration of the practice of the invention and are not meant to limit the scope of the invention in any way.